1. Field of the Invention
The present invention relates to wavelength division multiplexing transmission of optical signals, and, more particularly, to a wavelength division multiplexing transmission system, a wavelength division multiplexing transmission apparatus and a method for controlling wavelength division multiplexing transmission apparatus, allowing an economical redundant configuration to be achieved.
2. Description of the Related Art
FIG. 10 illustrates an example of configuration of a conventional wavelength division multiplexing (WDM) transmission apparatus.
As shown in FIG. 10, the conventional wavelength division multiplexing transmission system comprises nodes A, B, C, D and so on, and transmits optical signals by the following actions.
The nodes A and D each use a transponder (TPND) to convert into electricity an optical signal entered from a client (device) and 3R regenerate (Retime, Regenerate and Reshape) the signal for conversion into signals having a plurality of light wavelengths defined by ITU-T Grid (optical interface wavelength spacing according to ITU-T G. 692), use an optical multiplexer (OMUX) to multiplex and deliver the signals having a plurality of wavelengths as wavelength multiplexed signals, while at the same time, use a transmission amplifier (TXA) to amplify the delivered wavelength multiplexed signal for transmission through a transmission line.
Relay stations having integrated link adapters (ILAs), which are relay adapters, amplify and relay the wavelength multiplexed signals that flow through the transmission lines between the nodes A and B, and between the nodes C and D, respectively.
The nodes B and C each use a receive amplifier (RXA) to amplify the wavelength multiplexed signals transmitted through the transmission lines from the nodes A and D, respectively, and use an optical demultiplexer (ODMUX) to demultiplex the signals into signals having a plurality of wavelengths defined by the ITU-T Grid and to transmit the signals as optical signals to the respective opposite nodes B and C.
Each of the nodes B and C also comprises a back to back transponder (BBTPND) for converting into electricity the optical signals transmitted by the respective opposite nodes B and C, 3R regenerating and entering the signals into the OMUX.
Further, each of the nodes A, B, C and D comprises an optical supervisory channel (OSC) for extracting information on the number of multiple wavelengths in the wavelength multiplexed signal that is required to amplify the wavelength multiplexed signal (amplification using TXA and RXA) from a transmission line, respectively, sending it out to a different transmission line within the same node, and performing wavelength multiplexing of the signal into an optical signal from which information on the number of multiple wavelengths has been extracted.
Finally, the nodes A and D each amplify (RXA) the wavelength multiplexed signal entered through the transmission line from each of the nodes B and C, demultiplex (ODMUX) the signal into signals having a plurality of defined wavelengths, and deliver the signals as optical signals to the client through the TPND.
However, such a conventional configuration poses the problem that transmission cannot be achieved if any of the functions of the wavelength division multiplexing transmission system fails.
Thus, in order to solve the above problem, for example, Japanese Patent Laid-open No. 2004-32306 (Document 1) discloses a wavelength multiplexing transmission apparatus having a redundant configuration comprising an active system and a standby system, which monitors switching command information and a switching factor occurrence notification for the active system and the standby system originating from the failure of a function or the like, and is capable of switching to the standby system device if transmission of a signal cannot be carried out with the active system.
However, such a conventional wavelength division multiplexing transmission apparatus has the following problems.
A first problem is that the task of achieving a redundant configuration comprising an active system and a standby system is very difficult.
The reason is, when nodes are added to achieve a redundant configuration comprising an active system and a standby system, in order to control the nodes, it is necessary to consider a prescribed redundant configuration to predefine the implementation position and various settings of each node, leading to the control being very complicated. This made the task of achieving a redundant configuration by adding an extra new active system or standby system difficult.
A second problem is that when attempting to double the circuit with a wavelength division system that configures the active system and a wavelength division system that configures the standby system, the configuration becomes complicated, as a circuit for communicating control information is required.
The reason is, as each of the active system package and the standby system package monitors and keeps as information each other's circuit quality state to switch the circuit to the one with a better circuit quality between the TPNDs with a redundant configuration, a circuit is required for transmitting between the packages control information between the respective TPNDs that are implemented in the active system package and the standby system package.
A third problem is that, although the transmission distance is one that does not require regeneration relay of the optical signal for such reason as transmission is within the same station, hardware parts such as BBTPND are necessary, which is costly.
The reasons are as follows.
Normally, if the transmission distance is one that does not require regeneration and relay, a direct connection using an optical patch cord or the like is advantageous in terms of cost.
Further, because an optical amplifier has to adjust the level of excitation light in tune with the number of wavelengths of an optical signal to be entered, if the level of the excitation light in tune with the number of wavelengths is not entered, the level of the excitation light per wavelength becomes higher or lower than a design value; therefore, the targeted transmission performance would not be achieved. Therefore, the level of the excitation light to be entered per wavelength needs to be controlled by counting the number of wavelengths correctly.
However, in wavelength division multiplexing transmission, optical amplification with an erbium-doped optical fiber amplifier (EDFA) accumulates noise every time the relay is repeated. Therefore, if a signal is transmitted directly using an optical patch cord, the input unit of the OMUX sometimes detects the light power level, not of a signal but of noise, and erroneously recognizes the noise as a signal; therefore, it may not count the number of wavelengths of the optical signal correctly.
Besides, since the number of wavelengths of the optical signal cannot be counted correctly, the level of excitation light in tune with the number of wavelengths cannot be adjusted accurately; therefore, the quality of the transmission of the optical signal to be transmitted is significantly reduced.
For the above reasons, in order to count the number of wavelengths correctly, a BBTPND for converting an optical signal into an electrical signal and determining whether or not the signal is correct was needed, thus requiring a cost corresponding to the BBTPND.